Brush plating of one metal onto another is a technique which has been in use for many years. Early electrolytic technique, and the technique frequently employed even today, involves the immersion of a part to be plated in an electrolyte bath. The part forms a negative electrode and ions of a metal to be deposited on the part are deposited on the part by the flow of current from an anode through the electroltye, ions of the plating metal being in solution in the electroltye. The terms brush or selective plating are used when the electrolytic solution is wicked up or otherwise transferred from a bath through the fabric of a felt-like covering of a tool called a brush. The brush has a positive charge and the part to be plated, which is negatively charged, is brought into contact with the surface of the brush by a rubbing action until the portion contacted is plated electrolytically.
Early brush plating involved use of a hand held tool acting as anode, the tool having its surface impregnated with electrolyte. The surface was brushed or rubbed against the oppositely charged area to be plated until a satisfactory finish was achieved. More recent applications have provided a fixed brush against which interconnected parts are brushed while being moved lengthwise of the brush surface. Electrolytic solution is wicked upwardly onto the brush surface from a bath located beneath the part.
In batch plating, a variety of problems arise in attempting to confine the plating to a relatively small region of a part. It can be readily understood that if the part is simply immersed in the bath, all portions immersed are plated, even though it may be desired to plate only one portion, such as one side of a part. Furthermore, since the ions in solution in the plating bath are used up in that part of the solution between the anode and the part being plated, it is necessary to rather vigorously agitate the solution. This agitation makes it virtually impossible to precisely control the depth of plating on a partially immersed part, at least if relatively rapid plating and efficient use of electrolytic solution is desired.
Although selective plating including plating on one side of a part is possible using prior art brush plating techniques, plating is a relatively slow process inasmuch as the rate of plating is dependent upon the speed at which the electrolyte is wicked up through the brush to the region of the brush against which the part is rubbed.
An in-depth explanation of various plating techniques is found in Gold Plating Technology, Frank H. Reid & William Goldie, Electrochemical Publications Limited, Ayr, Scotland, 1974.
In accordance with the invention, high speed selective plating by the brush method is accomplished using a brush comprised of a molded body member formed of a porous, hydrophobic material covered by a felt-like material. The electrolytic solution, which contains the plating metal in solution, is distributed through a conduit located interiorly of the brush and passes outwardly through small pores in the hydrophobic material until it covers the felt-like surface material. An anode screen is disposed immediately beneath the brush surface and imposes a uniform charge on the metal ions in the plating solution as the solution passes through the openings of the screen. In a preferred embodiment, used for plating a variety of sheet metal electrical connector parts, interconnected in strip form, the brush has at least one elongated planar side, at least one elongated acutely radius edge and desirably has at least one elongated curved surface which has a substantially larger radius than the acutely radiused edge. Means are provided for rubbing or brushing negatively charged parts to be plated lengthwise over a selected elongated surface or edge of the brush. Important objects achieved by the invention are rapid plating, selectively, of one side of a part, of selected regions of a part or of curved portions of a part. A related object and advantage is the application of plating material in precisely controlled amounts. Another advantage of the invention is capability of precision control of the depth or width of the plated portion of a part. Plating quality is extremely uniform as fresh plating solution is continuously delivered to the entire surface of the brush in controlled amounts.
Another important object and advantage of the invention is the substantial savings in the amount of precious metals used in the plating of small metal parts.
Still another object achieved with the invention is a substantial increase in production rates in the plating of small interconnected electronic or electrical parts.
A related object of the invention is an increase in the efficiency of utilization of electrolytic plating solutions.
The above and other objects and advantages of the invention will become apparent from the detailed description of the invention and from the accompanying drawings.